The PackageManager does not display installed plugins. The installed plugin window is empty.
It shows plug-ins (packages) you install via the PackageManager.
Otherwise installed plug-ins will show up in the plug-in manager:
This arbitrary division may confuse Rhino users and gives the impression that the PackageManager is a half-baked idea. Neither of them knows that my old Rhinopolyhedra plugin should be upgraded.
No doubt there will be improvements in the future, and certainly some unification is warranted.
Have you uploaded your plug-in with Yak, or placed it on food4rhino.com with package manager support enabled? If not, then the Package manager can not know, otherwise it would tell the user of older versions if they got it via the package manager. And the plug-in manager has no mechanism for update notification.